In principle, oxide ceramics have a high application potential both as structural materials and for the thermal protection of combustion chambers and for hot-gas conducting components in airplane engines and stationary gas turbines. In such applications, temperatures of more than 1200° C. as well as thermomechanical stresses on the material occur from rapid temperature changes and local thermal loads.
Current oxide-ceramic fiber composite materials for engineering use under extreme thermal conditions, such as in gas turbines, either can be prepared only with a very high expenditure or, when the production expenditure is lower, have insufficient mechanical properties. The main problems involve the shrinking of the matrix during the drying and sintering process due to the low solids content in the slip employed.
Due to their brittleness, monolithic oxide ceramics are unsuitable for technical high-temperature use in safety-relevant cases. Therefore, since the 1990's, efforts have been made worldwide to develop oxide-ceramic materials which exhibit a damage-tolerant, i.e., quasiductile, behavior. One practicable possibility to prepare ceramic materials with sufficient “ductility” resides in the reinforcement by ceramic fibers, although the two components taken alone, i.e., the ceramic fiber and matrix, are inherently brittle.
Current oxide-ceramic fiber composite materials with sufficient mechanical properties for engineering use under extreme thermal conditions, such as the ceramic material WHIPOX® (wound highly porous oxide-ceramic matrix composite, DE 198 26 792 C2), are based on continuous oxide-ceramic fibers or cloths of the companies 3M (Nextel® 610 and Nextel® 720) or Nivity Company Ltd., Tokyo, Japan (R-960D). The high price of these aluminum silicate of aluminum oxide fibers highly determines the price of the ceramic composite prepared therefrom, since the volume proportion of the fibers is usually more than 30% by volume due to the preparation method.
Oxide-ceramic fiber composite materials which are mainly characterized by extreme thermoshock and thermofatigue resistance are being intensively developed and presented in various research facilities and companies. Such ceramics are preferably based on oxide-ceramic fiber cloths of the companies 3M® (Nextel® 610 and Nextel® 720), Nivity Company Ltd., Tokyo, Japan (R-960D), or Rath.
Sheet ceramic is an oxide fiber-reinforced oxide ceramic developed by the company Walter E. C. Pritzkow Spezialkeramik, Stuttgart, Germany. The fiber composite material consists of high-temperature resistant continuous fibers and matrices based on Al2O3, SiO2 and mullite. The material belongs to the class of oxide-ceramics matrix composites, briefly OCMC. The preparation of the structural members is effected with laminating technologies similar to those used in the preparation of fiber-reinforced plastic materials. With appropriate molds, processes and installations, sheets, tubes and complex thin-walled lightweight structures can be prepared. This oxide-ceramic fiber composite material, which is utilized in relatively small numbers, for example, in furnace construction, in combustion technology, energy or casting technology, can be employed at temperatures of below 1200° C. on a long-term basis and at temperatures of up to 1700° C. only on a short-term basis in accordance with the manufacturer. The tensile and bending strengths are on a low level.
COI Ceramics, Inc. (San Diego, Calif., USA; http://www.coiceramics.com) have developed an oxide-ceramic composite material mainly for use in commercial gas turbines. The preparation of the structural members is effected with laminating technologies similar to those used in the preparation of fiber-reinforced plastic materials, the final shaping of the green body being effected by vacuum technology. Thus, a fiber cloth is infiltrated with the slip and subsequently laid over a mold and dried on this mold in a vacuum step to form a green body, followed by sintering at about 1150° C. Based on aluminum silicate, this material reaches a tensile strength of up to about 365 MPa and an interlaminar shear strength of about 12 MPa for volume contents of the fibers of about 50% by volume. In this material, Nextel® 312, Nextel® 550, Nextel® 610 and Nextel® 720 fibers of 3M are employed.
The Materials Center Leoben presents (R. Simon, P. Supancic, Proceedings of the 28th International Conference & Exposition on Advanced Ceramics & Composites, Jan. 25-30, 2004, Cocoa Beach, Fla.; Verbundwerkstoffe, H.-P. Dregischer (Editor), Wiley-VCH Publishers, July 2003, pp. 298 to 303) the development and colloidal preparation of a novel oxide/oxide composite material. The preparation of the laminate is effected by infiltration of the ceramic cloths with the low-viscosity colloidal suspension having a low solids content in a classical wet-in-wet manual laminating method. The laminate is degassed and densified by means of the vacuum bag technique. The solidification of the laminate at room temperature takes from 24 to 48 hours. With a volume content of the fibers of 46 to 48%, a tensile strength of up to about 300 MPa and ILLS values for the interlaminar shear strength of about 14 MPa are achieved. The interlaminar shear strength can be measured according to DIN 65148 and is expressed by the quotient of the force resulting in a break failure within the shear surface to the shear surface.
For the preparation of oxidic CMCs, the Fraunhofer ISC (A. Rudinger, W. Glaubitt, 15. Symposium Verbundwerkstoffe und Werkstoffverbunde, Apr. 6-8, 2005, Universität Kassel) develops binder systems and filler powders based on a supramolecular organic precursor. By dip coating with a coating sol and subsequent thermal processing, an intermediate layer is applied to the fibers. The plastification of the binder systems at temperatures of from 100 to 140° C. enables post-densification of the CMC green bodies, which results in increased fiber contents of the CMCs within a range of about 30% by volume. The tensile strength of these ceramic composites is 152 MPa (0°/90°), their three-point bending strength is about 250 MPa, and their ILSS is 4 MPa.
DE 198 26 792 A1 describes a highly thermally resistant and oxidation-resistant fiber composite material made of oxide-ceramic fibers, and a method for the preparation thereof.
EP 02 60 867 A1 describes a furnace lining made of a fiber-containing ceramic material.
U.S. Pat. No. 6,472,059 B2 describes a sandwich-like composite of long fiber CMC (ceramic matrix composite) and short fiber CMC. In the process described, a polymer-derived green body is ceramized by pyrolysis. The bonding of the long and short fiber components is effected in the wet state.
U.S. Pat. No. 5,198,282 A and U.S. Pat. No. 5,376,598 A describe a ceramic insulation composite material. The long fiber component therein has a high density and may even be glass-like. The matrix contains whiskers, which are problematic for health reasons. The bonding of the long and short fiber components is effected in the wet state.
U.S. Pat. No. 6,733,907 D2 describes a composite of a ceramic support structure and ceramic heat insulation layer. The heat insulation layer has a higher temperature resistance than the long fiber reinforced support structure and protects the latter from too high thermal loads. A precondition of this concept is backside cooling and a sufficient heat transport by the support structure.
DE 10 2004 049 406 A2 describes a multilayer shaped article made of high-temperature resistant, chemically resistant and mechanically damage-tolerant ceramic materials, and a process for the preparation of the shaped article.
A disadvantage of the prior art is the high fiber content of the ceramic fiber composite materials of typically more than 30% by volume, which results from the usual preparation technology. On the one hand, it results from the particle size distribution of the oxide-ceramic powders employed (for example, aluminum silicate or aluminum oxide powder) which, in the form of a suspension, are infiltrated into the fiber cloths or individual continuous rovings (bundles of individual filaments). On the other hand, the fiber content is also highly affected by the dynamic viscosity of the suspension. However, sufficient amounts of small particles are necessary for a sufficient sintering activity in a temperature range which does not result in damage to (grain growth) the fibers. A high amount of small particles typically results in an increased viscosity. However, a low dynamic viscosity of the suspension is a basic precondition for a complete infiltration of the fiber bundles or fiber cloths. Due to the high fiber content, a contact between the fibers may occur, which results in a deterioration of the mechanical properties. However, if the solids content is too low in order to achieve a low dynamic viscosity of the slip, then an uncontrollably high volume proportion of the fibers occurs.
The preparation of previously known oxide-ceramic composites is effected by the infiltration of fiber cloths with a usually complex slip, which mostly contains elevated amounts of binders, requiring the burning of the green body. In addition, an expensive and time-intensive vacuum and drying technology is employed. Therefore, the resulting materials are very expensive because the production thereof often takes a few days (for example, COI Ceramics, Inc.), or because the starting materials employed are already extremely expensive (Materials Center Leoben, Austria).
However, when the ceramic composites can be prepared relatively easily, such as sheet ceramics (Walter E. C. Pritzkow Spezialkeramik, Stuttgart, Germany), the material characteristics, such as modulus of elasticity, tensile strength or bending strength, are within a range which excludes many fields of application.